Manufacture of semiconductor wafers uses a complex and precise lithographic process. Quality control and analysis of the lithographic process through each stage of wafer manufacture is critical to ensure the quality and reliability of the final product. Inconsistencies within the lithographic process are undesirable and early detection and correction of these inconsistencies saves a significant amount of time and expense for the manufacturer. For example, if problems with lithographic line and space definition are identified after resist development and before etching, wafers can be reworked. Similarly, if lithographic issues can be identified soon after etching, process adjustments can be made or replacement wafers started many weeks sooner than required if problems are detected after wafer completion and electrical testing.
Typically, wafers are viewed and measured using a critical dimension-scanning electron microscope (CD-SEM) technique, wherein an electron microscope is used to image pattern features within a small portion of a wafer and measurements of the pattern features are made. However, the CD-SEM technique images only a small portion (e.g., less than 1 μm square at a time without distortion) of the wafer, and requires many days or weeks of continuous imaging and analysis to evaluate the entire wafer. Thus, manufacturers typically image a few sample points of a wafer using CD-SEM to evaluate the wafer as a whole. Such limited sampling, however, may not be representative of the wafer as a whole. Further, CD-SEM typically makes a single critical dimension (CD) measurement of each imaged pattern, which may give skewed results because of localized process variation across a wafer. That is, the CD measurement made by CD-SEM may not be particularly representative of the wafer as a whole because of the localized process variation.
In the view of measurement accuracy of CD-SEM devices, errors in line width measurements of a pattern are near to an allowable error of control values of process conditions. It is necessary to improve accuracy of the measurement of the line width of the pattern by using an average of measurements of line widths of patterns.